In zinc-air cells, air enters the cell through port(s) in the cell which are immediately adjacent to a cathode assembly. The air diffuses into an air cathode sub-assembly where the oxygen is reacted. This air cathode sub-assembly generally consists of mixtures of activating chemicals supported by a complex physical structure. The air cathode sub-assembly also slows the diffusion of other gases, particularly carbon dioxide and water vapor, through the electrode to the reaction site. Other gases in air, particularly water vapor, have a profound limiting effect on the capacity of the cell.
The activated oxygen diffuses through a separator, a moisture barrier usually of a plastic-like material impervious to liquids such as the alkaline electrolyte, and reacts with the water in the electrolyte consuming electrons and producing hydroxide ions. These ions then oxidize the metallic zinc, generally producing two electrons for each atom of zinc reacted. Such air cathode electrochemical cells are well known, and are more fully discussed in references such as U.S. Letters Pat. Nos. 3,149,900 (Elmore and Tanner) and 3,276,909 (Moos).
It is well known that the limit to the efficiency of zinc-air cells below the theoretical rate capacity is due to two different factors. Initially, the cell efficiency is limited by the cathode. The rate of mass transport of the oxygen into the cathode limits the reaction of the oxygen with water in the alkaline environment. The present invention, however, is not directed to electrochemical cell rate capacity limitations due to the cathode.
The present invention is directed to the reduction in cell efficiency which occurs after the zinc-air cell has been substantially discharged. This limitation in rate capacity is related to events occurring in the zinc anode during discharge. As zinc is oxidized, a zinc oxide layer forms on the surface of the metallic zinc. Zinc oxide is a known poor conductor of electricity. As the cell continues to discharge, the zinc oxide layer grows. As the zinc oxide layer grows, the internal conductivity and therefore the rate capacity decreases. The rate capacity is further limited by side reactions which compete with the main reaction for the metallic zinc. These side reactions consume zinc without producing any free electrons. A very rate limiting side reaction is: ##STR1##
Water, the solvent of the electrolyte, is a readily available reactant. The production of the hydrogen gas quickly increases the internal pressure of the zinc-air cell, which can cause the seals of the cell to fail, resulting in a leakage of electrolyte. This rate limiting reaction appears to be catalyzed by carbon. Therefore, carbon, which is often present in the cathode assembly, must be isolated from the zinc reduction zones. The presence of carbon in the zinc anode assembly, even in trace amounts, greatly increases the rate of hydrogen gas formation.
The objective of the present invention is to extend the capacity of zinc-air button cell by lessening the effects of the rate limitations which originate in the zinc anode. This objective and other subsidiary objectives are achieved by the practice of the present invention.